Queen,
All three of your ships are the Millennium class so they will only differ in decor. I've been on both the Millennium and the Summit. I am trying the Infinity (another M-class) in Alaska this coming June. I have a friend who really loves the Constellation. Yes, longer cruises would tend to not have as many children. It also depends on the time frame you are looking at going. Bermuda is a wonderful cruise. It has several beautiful beaches. Some have bigger waves, and smaller beaches like Tobacco beach are more protected. The hop on/off bus is great in Bermuda and you can do a lot by using the bus. It is also good for the ferry across the harbor from Hamilton to King's Wharf. You can get walking maps from the tourist center's which are close to the ports and walk all over the town too and see a lot of history.